The invention concerns a method for commutating a brushless motor, supplied with electrical energy from a DC intermediary circuit via a multi-phase inverter, by which a first current value and a second current value are determined, and the duration of a commutation interval is set in dependence of these current values. Further, the invention concerns a power supply for a brushless motor, the motor being connected with a DC intermediary circuit via an inverter, comprising a control unit with an input connected to the DC intermediary circuit.
With inverter controlled motors the individual motor phases must be turned on with optimum timing. Turn on must take place in correspondence with the counter voltage or counter-electromotive force (back-EMF) produced by the rotor, so that the motor does not get out of phase or timing, i.e. jumps, or even stops. This is especially important for motors, the rotors of which are equipped with permanent magnets, as here there are no possibilities for changing the flux produced by the rotor. Thus it is known to measure the induced counter-electromotive force in the windings and to use it to control the speed and to determine the moment of commutation. Hereby a measurement of the rotor position and speed can be avoided. This sensorless control is effective, but considerable costs are implied in realising it. Normally three voltage sensors (or a number corresponding to the number of phases) are required in the motor cables, which will increase the costs for the production and operation of such a motor due to the plurality of components.
Thus, it has become widely used to avoid the position and speed feed-back, and to measure the current in the intermediary circuit instead. The motor can also be controlled by means of this information. This principle can be used for both AC and three-phase current synchronous motors, and for brushless DC motors as well.
With brushless DC motors it is known to change the commutation time dynamically as a function of the current in the intermediary circuit. For this purpose the current is measured and converted to a processable parameter. This parameter is compared with a predetermined reference parameter. In dependence of the result of this comparison, the commutation interval is either kept constant or reduced or prolonged. Here commutation interval means the time between the individual commutations.
U.S. Pat. No. 5,420,492 describes a design, in which the commutation frequency, i.e. the frequency of commutations, is changed in dependence of the current over time, i.e. in dependence of the current waveshape profile. This current waveshape profile is compared with a pre-set profile. The contour of the current profile depends on whether the commutation time was correct, too early or too late. In the solution revealed in U.S. Pat. No. 5,420,492 the slope of the current is determined. When the moment of commutation occurs too early, i.e. is ahead of the rotor, the slope will be too flat. When the commutation occurs too late, the slope will be too steep. If the waveshape profile is not correct, the commutation interval is either reduced or prolonged, and a new test is made, until the correct profile, and thus the correct commutation interval, has been set. This is done via a change of the commutation frequency.
However, it is difficult to determine the optimum profile of the current, i.e. the optimum slope or the optimum relation between the two current values. The values can be determined empirically for the unloaded motor and then be taken from a look-up table during the operation. For a loaded motor, however, this determination is relatively difficult, as the kind and size of the load is not known. In dynamic systems with heavily varying loads, a correspondingly large number of reference parameters would be necessary.
This is for instance the case, when the motor is driving a compressor. In this case it is difficult to obtain an optimum control of the operation with the known method.
Piston compressors are for example used in refrigeration systems, in which they drive a refrigerant gas through a capacitor and an evaporator. Here the load torque is varying over a cycle, i.e. over a piston stroke. A typical load curve starts in the lower dead point with a small torque of approximately 0 Nm. The closer the piston comes to the upper dead point, the faster the torque increases. In most cases this increase is not linear. The outlet valve of the compressor opens in the range of the upper dead point, or somewhat earlier, e.g. at 150.degree.. The load torque then decreases very rapidly and steeply. Normally this load-torque-decrease is not linear either.
When a speed control is not used, the motor speed changes during the load changes. This involves the risk that the motor runs very irregularly and that the rotor looses its synchronisation with the rotary field produced by the inverter and stops or jumps.
From P. S. Frederiksen et al. "Comparison of two energy optimising techniques for PM-machines", Alborg University 1994, it is known to measure the amplitude of the intermediary circuit current, and from this derive information for the stabilisation and prevention of oscillations in the rotational speed. In this connection a first proposal is based on the fact that the medium value of the intermediary circuit current in a stationary operation point is minimised and the impressed stator voltage is set so that a power minimum is reached. The second proposal controls a phase modulated machine in dependence of the profile of the intermediary circuit current, which depends on the power factor. Also in this case the stator voltage of the phase modulated machine is set so that an optimum power factor and efficiency are reached. This solution is made so that the measured intermediary circuit current is led through an analogue low frequency filter to remove the harmonic components. However, this means that the solution is not suited for the control of a compressor, as the intermediary circuit current changes heavily during a working cycle of the compressor, and the sensed currents do not give the true picture of the load of the motor. Further, Frederiksen et al. are sampling the intermediary circuit current with a fixed frequency and independently of the phase angle of the motor voltage.